The present invention relates to systems for providing automatic frequency control in radio frequency communication systems and, more specifically, to a system for compensating for frequency offset in a digital mobile radio frequency communication system through automatic frequency control.
The cellular telephone industry has made phenomenal strides in commercial operations in the United States as well as the rest of the world. Growth in major metropolitan areas has far exceeded expectations and is rapidly outstripping system capacity. If this trend continues, the effects of this industry""s growth will soon reach even the smallest markets. Innovative solutions are required to meet these increasing capacity needs as well as maintain high quality service and avoid rising prices.
Digital communication systems, at their most basic level, provide for the transmission and reception of electronic messages between and among communication partners. The transmissions are effected through transmitters that modulate or encode the message and transmit the message in analog form for passage across a channel. At the receiver, the analog signal is converted back to the digital data of the message. Although paired transmitters and receivers are assigned to the same carrier, and the receiver is designed to perfectly demodulate (or decode) the modulated, transmitted signal, frequency offsets, or deviations, in the received signal may occur because of imperfections of oscillators and frequency synthesizers in the receiver. The frequency offset becomes, with time, a growing phase drift, which compromises the ability of the receiver to accurately and efficiently receive the transmitted messages. Therefore, in order to accurately detect sent information with minimal reception performance loss, the frequency offset of the received signal should be taken into consideration in the receiver design and compensated for during equalization.
Within the standards set for mobile radio frequency (xe2x80x9cRFxe2x80x9d) communication systems, frequency offsets of up to several hundred hertz are allowed. For example, a system conforming to the Global System for Mobile Communication (xe2x80x9cGSMxe2x80x9d) has a channel spacing of 200 KHz, thereby providing some tolerance for frequency offset without encountering the risk of receiving the wrong channel of transmitted data or receiving the transmitted data incorrectly. In contrast, however, a digital satellite communications system may use a channel spacing of only 5 KHz, with a correspondingly tight tolerance for frequency offset of received data.
In digital cellular telephones, automatic frequency control (xe2x80x9cAFCxe2x80x9d) is commonly used in RF receivers to keep the receiver locked on a particular frequency, despite imperfect component stability that would otherwise result in frequency drift. In contemporary digital communication systems, AFC is commonly based on second order digital phase-locked loop (xe2x80x9cPLLxe2x80x9d) filters that implement phase offset compensators to enable reliable communications. Such conventional AFC systems are used, for example, in current ANSI-136 systems, and effectively attempt to determine the phase error, to eliminate the phase drift. For a more thorough discussion of PLL filters and their use to determine phase error, see W. Lindsey and C. Chie, xe2x80x9cA Survey of Digital Phase-Locked Loops,xe2x80x9d 69 Proc. IEEE 410-31 (April 1981); K. J. Molnar and G. E. Bottomley, xe2x80x9cAdaptive Array Processing MLSE Receivers for TDMA Digital Cellular/PCS Communications,xe2x80x9d 16 IEEE J. Selected Areas in Comm. 1340-51 (October 1998).
More specifically, the impact of phase drift on an actual sampled, received data signal r(n) as a function of time index n=1, 2, 3, . . . , is commonly modeled as:
r(n)=ejxcfx86(n)z(n)+v(n)xe2x80x83xe2x80x83(1)
where v(n) is additive noise from the transmission, and z(n) represents the desired signal, i.e., the signal carrying the transmitted data, and is represented by:                                           z            ⁡                          (              n              )                                =                                    ∑                              k                =                0                                            L                -                1                                      ⁢                                          h                ⁡                                  (                  k                  )                                            ⁢                              s                ⁡                                  (                                      n                    -                    k                                    )                                                                    ,                            (        2        )            
where h(k) is a set of channel estimates for the channel across which the signals have been transmitted and L is the number of taps for the channel, and where s(nxe2x88x92k) is the transmitted signal associated with the time index (nxe2x88x92k). xcfx86(n) of equation (1) is the phase drift, given by:
xcfx86(n)=(nxe2x88x92n0)xcfx890,xe2x80x83xe2x80x83(3)
with xcfx890 being the frequency offset and n0 being the reference sample data index, defining the sample position where xcfx86(n0)=0. More basically, the phase drift can be viewed as the frequency offset multiplied by time. This phase drift will be added, if uncorrected, to the phase of the desired signal, z(n). By estimating the phase drift as xcfx86xe2x80x2(n), the received signal can be phase compensated as:
xe2x80x83ŕ(n)=r(n)exe2x88x92jxcfx86xe2x80x2(n)xe2x80x83xe2x80x83(4)
If the estimate of the phase drift is close to xcfx86(n), and if the signal-to-noise ratio is sufficiently high, then the resultant signal ŕ(n) should be a good signal from which to detect the transmitted data. On the other hand, if nothing is done to compensate for errors in the estimates of the phase drift, the phase errors will degrade the ability of the receiver to determine the transmitted data. The detection of the sent data is thus based on phase-compensated data signals, with the aim of the AFC to provide accurate estimates of the phase drift. Conventional digital AFC systems provide this compensation as follows:
At each increment of the time index n, a phase error estimate is first calculated according to:
xcfx86error(n)=arg(ejxcfx86xe2x80x2(n)r*(n)ź(n))xe2x80x83xe2x80x83(5)
where arg( ) denotes phase, * denotes conjugation, ź(n) denotes an estimate of the desired signal z(n), and xcfx86xe2x80x2(n) is an estimate of the phase drift xcfx86(n). Then, using the calculated phase error estimate, a new estimate of the frequency offset and a new estimate of the phase drift are determined by a second order filter according to:
{circumflex over (xcfx89)}0(n+1)={circumflex over (xcfx89)}0(n)+K1xcfx86error(n)xe2x80x83xe2x80x83(6a)
xcfx86xe2x80x2(n+1)=xcfx86xe2x80x2(n)+{circumflex over (xcfx89)}0(n+1)+K2xcfx86error(n)xe2x80x83xe2x80x83(6b)
in which K1 and K2 are two constant filter parameters, where {circumflex over (xcfx89)}0(n+1) denotes an updated estimate of the frequency offset xcfx890, and where. xcfx86xe2x80x2(n+1) denotes an updated estimate of the phase drift. This scheme requires the estimation of the desired signal, or ź(n). The choice of parameters K1 and K2 is a trade-off between fast convergence to the true frequency offset and sensitivity to noise. The parameters are set by prior simulations of data transmissions, using an upper estimate of the frequency offset, xcfx890, and an estimate of the anticipated noise, v(n). For example, when considering symbol spaced sampled received signals in a GSM system, typical values for K1 and K2 are less than 0.05 and 0.15, respectively.
Referring now to FIG. 1, there is shown a block diagram of an example of an automatic frequency control system. A data signal, r(n), received across a Channel 100 is directed to a Detector 102 to determine the transmitted data. The Detector 102 includes, for example, a Channel Estimator 104, an Equalizer 106, and an AFC 108. The received data signal, r(n), is directed to the Channel Estimator 104, where values of h(k) (channel estimates) of equation (2) are determined by comparing the training sequence within each received burst of data against the known data sequence that corresponds thereto (see also FIG. 3b). The received data signal, r(n), is also directed to both the Equalizer 106 and the AFC 108. The Equalizer 106 produces the desired signal estimate, ź(n), for input into the AFC 108, and the soft output data, for input into the Decoder 110. The desired signal estimate, ź(n), is input to the AFC 108 for use in equation (5) above to produce the estimate of the phase drift, xcfx86xe2x80x2(n), through application of equation (6b). The phase drift xcfx86xe2x80x2(n) is, in turn, utilized in equation (4) to calculate the phase compensated signal, ŕ(n), in the Equalizer 106. In such a manner, the data signal, r(n), is phase corrected in the Equalizer 106 and used to generate the soft values, or data, for use by the Decoder 110. The iteration of data through the Detector 102 continues until the end of the received data batch, at which time the phase compensated received signal, ŕ(n), has been fully processed by the Equalizer 106 to produce soft output data for decoding by the Decoder 110 and for further processing as received data.
The problem with this conventional scheme is that if values of the constant filter coefficients K1 and K2 are set for fast convergence, the resulting phase estimate will not be very accurate. However, in the receiver it is desirable to provide a rapidly convergent phase estimate for accurate detection of the transmitted information. Accordingly, it would be desirable to provide an automatic frequency control system that quickly and reliably compensates for the frequency offset at the reception point for digital mobile radio communications.
The preferred embodiments of the present invention overcome the problems associated with existing mechanisms for providing automatic frequency control for RF receivers in digital communication systems.
The present invention is directed toward a method and system for providing automatic frequency control (AFC) for received data in a mobile, radio frequency communication system. AFC systems and techniques according to the present invention provide automatic frequency control based on, among other things, a first order filtering process that employs a time variant coefficient.
According to an exemplary embodiment of the present invention, a reference sample index is selected as a function of the taps of the channel across which the data will be transmitted and the amount of training data with which the channel is estimated. The system then calculates an estimate of the desired, or transmitted, signal. Next, a scaled phase error estimate associated with the desired signal estimate is determined. The scaled phase error estimate is multiplied by a single order filter coefficient that varies as a function of time. The product of this multiplication is then added to a frequency offset estimate, thereby updating the frequency offset estimate. This process continues until the entire received batch of data signals has been processed. By calculating a scaled phase error estimate in this way, the present method and system for compensating for frequency offset provides rapid convergence without excessive amplification of the noise present in the received signal, thereby producing accurately compensated, received data.
The present inventive system differs from prior systems of automatic frequency control by employing a first order filtering process with a time variant scaling function that varies based on an estimate of the desired signal, instead of a second order filtering process that relies on constant scaling quantities. Thus, the present inventive AFC system has the advantage of being less complex than conventional AFC systems by utilizing only a single order filter and a single filter parameter, K. Also, the present invention provides better performance than conventional AFC""s by ensuring a faster convergence for the frequency offset estimate, {circumflex over (xcfx89)}0(n), and by ensuring that the variance of the estimation errors tends to zero as time goes to infinity. In addition, the present inventive system eliminates the need to explicitly compute the phase of the received signal, which is a computationally intensive task.